mm/page-writeback.c: use div64_ul() for u64-by-unsigned-long divide

mm/page-writeback.c: use div64_ul() for u64-by-unsigned-long divide

The two variables 'numerator' and 'denominator', though they are
declared as long, they should actually be unsigned long (according to
the implementation of the fprop_fraction_percpu() function)

And do_div() does a 64-by-32 division, while the divisor 'denominator'
is unsigned long, thus 64-bit on 64-bit platforms.  Hence the proper
function to call is div64_ul().

Link: http://lkml.kernel.org/r/20200102081442.8273-3-wenyang@linux.alibaba.com
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>